1. Field of the Invention
The present invention relates a liquid crystal display device and a method for producing the same. In particular, the present invention relates to a liquid crystal display device having wide viewing angle characteristics and a method for producing the same.
2. Description of the Related Art
Various display modes have been used for a liquid crystal display device. For example, as liquid crystal display devices utilizing an electro-optical effect, there are twisted nematic (TN) and supertwisted nematic (STN) liquid crystal display devices using a nematic liquid crystal, which have been put into practical use; and a liquid crystal display device using a ferroelectric liquid crystal. These devices require a polarizer and alignment treatment. In addition, there are liquid crystal display devices utilizing a dynamic scattering (DS) effect or a phase change (PC) effect. These devices utilize light scattering caused by the liquid crystal and do not require polarizers.
In recent years, a polymer dispersed liquid crystal mode (PDLC mode) has been proposed. In this mode, a polarizer and alignment treatment are not required, and a transparent state and an opaque state are electrically switched utilizing the birefringenece of liquid crystal. The basic principle of the PDLC mode is as follows: The PDLC has a structure in which liquid crystal phases are dispersed in a polymer matrix. Materials for the liquid crystal and the polymer are selected so that the ordinary light refractive index of the liquid crystal is identical with the refractive index of the polymer matrix. When a voltage is applied to the PDLC, the liquid crystal molecules are oriented to display a transparent state; when no voltage is applied to the PDLC, the orientation of the liquid crystal molecules are disturbed, so that light incident upon the PDLC is scattered to display an opaque state.
Various methods for producing the PDLC are known. Japanese National Publication No. 58-5-501631 discloses the encapsulation of liquid crystal in a polymer; Japanese National Publication No. 61-502128 discloses that liquid crystal is mixed with photocurable or thermosetting resin and the resin is cured, thereby phase-separating the liquid crystal to form liquid crystal droplets in the resin; and Japanese Laid-Open Patent Publication No. 59-226322 discloses that a mixture including a polymer, liquid crystal, and a solvent for dissolving the polymer and liquid crystal is prepared, and the solvent is removed, thereby forming a phase separation of the polymer and liquid crystal.
A liquid crystal display device performing a non-scattering mode display, using the PDLC and a polarizer has also been proposed. Japanese Laid-Open Patent Publication No. 5-27242 discloses a method for producing such a liquid crystal display device: A mixture of liquid crystal and photocurable resin is irradiated with UV-rays to phase-separate the liquid crystal and the polymer, thereby forming a polymer network structure in a liquid crystal layer (i.e., a random matrix in which liquid crystal regions are arranged in a matrix in the polymer material). According to this method, the polymer network is also formed in pixels, so that transmittance and contrast are low.
In Japanese Patent Application No. 5,30996, the inventors of the present invention have proposed a method for producing a polymer dispersed liquid crystal display device as described below.
According to this method, a mixture of a liquid crystal material, photocurable resin, and photoinitiator is injected between facing substrates. A photomask is placed so as to cover pixel portions. UV-rays are irradiated to the cell under this condition. As a result, liquid crystal regions are formed in the pixel portions and a polymer is formed in the other portions. Unlike the method disclosed in Japanese Laid-Open Patent Publication No. 5,27242, since a polymer network is not formed in the pixel portions, there is no decrease in contrast characteristic. In addition, by radially or randomly arranging liquid crystal domains in the liquid crystal regions, viewing angle characteristics of the liquid crystal display device can be markedly improved.
However, the above method has a problem that a liquid crystal display device to be obtained has an insufficient contrast characteristic.
First, a common problem in a method for producing active matrix type and simple matrix type liquid crystal display devices will be considered. Unwanted light is incident upon the pixels during light irradiation to form an unwanted cured polymer in the pixels; as a result, the liquid crystal display device obtained has a low contrast characteristic. The formation of the unwanted cured polymer in the pixels is caused by the photomask which is not designed in an appropriate shape and size and by the low positioning precision between the photomask and the liquid crystal cell. Alternatively, light diffraction plays a part in the formation of the unwanted curd polymer in the pixels. More specifically, there is a distance (corresponding to the thickness of a transparent substrate) between a photomask or a light-shielding member formed on the substrate and a mixture to be subjected to light irradiation. Thus, if the pattern of the photomask or the light-shielding member is small, light, which is irradiated to the mixture of liquid crystal and a photocurable polymer sandwiched between facing substrates for producing the PDLC, is diffracted to come in the pixels. As a result, the cured polymer is formed in the pixels.
Furthermore, when the above liquid crystal display device is produced using a substrate on which an active element such as a TFT is formed, UV-rays are directly irradiated to the active element to cause the deterioration of the electronic characteristics of the active element. As a result, the contrast characteristic of the liquid crystal display device is deteriorated.